1. Field of the Invention
This invention relates to precision motion stages and more specifically to a stage suitable for use in a photolithography machine and especially adapted for supporting a reticle.
2. Description of the Prior Art
Photolithography is a well known field especially as applied to semiconductor fabrication. In photolithography equipment a stage (an X-Y motion device) supports the reticle (i.e., mask) and a second stage supports the semiconductor wafer, i.e. the work piece being processed. Sometimes only a single stage is provided, for the wafer or the mask.
Such stages are essential for precision motion in the X-axis and Y-axis directions and often some slight motion is provided for adjustments in the vertical (Z-axis) direction. A reticle stage is typically used where the reticle is being scanned in a scanning exposure system, to provide smooth and precise scanning motion in one linear direction and insuring accurate, reticle to wafer alignment by controlling small displacement motion perpendicular to the scanning direction and a small amount of "yaw" (rotation) in the X-Y plane. It is desirable that such an X-Y stage be relatively simple and be fabricated from commercially available components in order to reduce cost, while maintaining the desired amount of accuracy. Additionally, many prior art stages include a guide structure located directly under the stage itself. This is not a desirable in a reticle stage since it is essential that a light beam be directed through the reticle and through the stage itself to the underlying projection lens. Thus a stage is needed which does not include any guides directly under the stage itself, since the stage itself must define a fairly large central passage for the light beam.
Additionally, many prior art stages do not drive the stage through its center of gravity which undesirably induces a twisting motion in the stage, reducing the frequency response of the stage. Therefore there is a need for an improved stage and especially one suitable for a reticle stage.